diff --git a/plugins/Fediverse/plugin.py b/plugins/Fediverse/plugin.py
index a438cf189..2bfcfa583 100644
--- a/plugins/Fediverse/plugin.py
+++ b/plugins/Fediverse/plugin.py
@@ -122,7 +122,7 @@ class Fediverse(callbacks.PluginRegexp):
     def __init__(self, irc):
         super().__init__(irc)
         self._startHttp()
-        self._actor_cache = utils.structures.TimeoutDict(timeout=600)
+        self._actor_cache = utils.structures.ExpiringDict(timeout=600)
 
     def _startHttp(self):
         callback = FediverseHttp()
diff --git a/src/utils/structures.py b/src/utils/structures.py
index 55aa7ee6f..31c810355 100644
--- a/src/utils/structures.py
+++ b/src/utils/structures.py
@@ -459,7 +459,8 @@ class CacheDict(collections.abc.MutableMapping):
 
 
 class TimeoutDict(collections.abc.MutableMapping):
-    """A dictionary that may drop its items when they are too old.
+    """An efficient dictionary that MAY drop its items when they are too old.
+    For guaranteed expiry, use ExpiringDict.
 
     Currently, this is implemented by internally alternating two "generation"
     dicts, which are dropped after a certain time."""
@@ -533,6 +534,81 @@ class TimeoutDict(collections.abc.MutableMapping):
         return len(set(self.new_gen.keys()) | set(self.old_gen.keys()))
 
 
+class ExpiringDict: # Don't inherit from MutableMapping: not thread-safe
+    """A dictionary that drops its items after they have been in the dict
+    for a certain time.
+
+    Use TimeoutDict for a more efficient implementation that doesn't require
+    guaranteed timeout.
+    """
+    __slots__ = ('_lock', 'd', 'timeout')
+    __synchronized__ = ('_expire_generations',)
+
+    def __init__(self, timeout, items=None):
+        expiry = time.time() + timeout
+        self._lock = threading.Lock()
+        self.d = {k: (expiry, v) for (k, v) in (items or {}).items()}
+        self.timeout = timeout
+
+    def __reduce__(self):
+        return (self.__class__, (self.timeout, dict(self)))
+
+    def __repr__(self):
+        return 'ExpiringDict(%s, %r)' % (self.timeout, dict(self))
+
+    def __getitem__(self, key):
+        with self._lock:
+            try:
+                (expiry, value) = self.d[key]
+                if expiry < time.time():
+                    del self.d[key]
+                    raise KeyError
+            except KeyError:
+                raise KeyError(key) from None
+
+            return value
+
+    def __setitem__(self, key, value):
+        with self._lock:
+            self.d[key] = (time.time() + self.timeout, value)
+
+    def clear(self):
+        with self._lock:
+            self.d.clear()
+
+    def __delitem__(self, key):
+        with self._lock:
+            del self.d[key]
+
+    def _items(self):
+        now = time.time()
+        with self._lock:
+            return [
+                (k, v) for (k, (expiry, v)) in self.d.items()
+                if expiry >= now]
+
+    def keys(self):
+        return [k for (k, v) in self._items()]
+
+    def values(self):
+        return [v for (k, v) in self._items()]
+
+    def items(self):
+        return self._items()
+
+    def __iter__(self):
+        return (k for (k, v) in self._items())
+
+    def __len__(self):
+        return len(self._items())
+
+    def __eq__(self, other):
+        return self._items() == list(other.items())
+
+    def __ne__(self, other):
+        return not (self == other)
+
+
 class TruncatableSet(collections.abc.MutableSet):
     """A set that keeps track of the order of inserted elements so
     the oldest can be removed."""
diff --git a/test/test_utils.py b/test/test_utils.py
index b45623152..f4ba2c0a8 100644
--- a/test/test_utils.py
+++ b/test/test_utils.py
@@ -1207,6 +1207,57 @@ class TestTimeoutDict(SupyTestCase):
         d2['baz'] = 'qux'  # does not move it (7 seconds old)
         self.assertEqual(d1, d2)
 
+
+class TestExpiringDict(SupyTestCase):
+    def testInit(self):
+        d = ExpiringDict(10)
+        self.assertEqual(dict(d), {})
+        d['foo'] = 'bar'
+        d['baz'] = 'qux'
+        self.assertEqual(dict(d), {'foo': 'bar', 'baz': 'qux'})
+
+    def testExpire(self):
+        d = ExpiringDict(10)
+        self.assertEqual(dict(d), {})
+        d['foo'] = 'bar'
+        timeFastForward(11)
+        d['baz'] = 'qux'
+        self.assertEqual(dict(d), {'baz': 'qux'})
+
+        timeFastForward(11)
+        self.assertEqual(dict(d), {})
+
+        d['quux'] = 42
+        self.assertEqual(dict(d), {'quux': 42})
+
+    def testEquality(self):
+        d1 = ExpiringDict(10)
+        d2 = ExpiringDict(10)
+        self.assertEqual(d1, d2)
+
+        d1['foo'] = 'bar'
+        self.assertNotEqual(d1, d2)
+
+        timeFastForward(5)  # check they are equal despite the time difference
+
+        d2['foo'] = 'bar'
+        self.assertEqual(d1, d2)
+
+        timeFastForward(7)
+        self.assertNotEqual(d1, d2)
+        self.assertEqual(d1, {})
+        self.assertEqual(d2, {'foo': 'bar'})
+
+        timeFastForward(7)
+        self.assertEqual(d1, d2)
+        self.assertEqual(d1, {})
+        self.assertEqual(d2, {})
+
+        d1['baz'] = 'qux'
+        d2['baz'] = 'qux'
+        self.assertEqual(d1, d2)
+
+
 class TestTruncatableSet(SupyTestCase):
     def testBasics(self):
         s = TruncatableSet(['foo', 'bar', 'baz', 'qux'])