diff --git a/astroid/nodes/as_string.py b/astroid/nodes/as_string.py
--- a/astroid/nodes/as_string.py
+++ b/astroid/nodes/as_string.py
@@ -36,6 +36,7 @@
         MatchSingleton,
         MatchStar,
         MatchValue,
+        Unknown,
     )
 
 # pylint: disable=unused-argument
@@ -643,6 +644,9 @@ def visit_property(self, node):
     def visit_evaluatedobject(self, node):
         return node.original.accept(self)
 
+    def visit_unknown(self, node: "Unknown") -> str:
+        return str(node)
+
 
 def _import_string(names):
     """return a list of (name, asname) formatted as a string"""

diff --git a/tests/unittest_nodes.py b/tests/unittest_nodes.py
--- a/tests/unittest_nodes.py
+++ b/tests/unittest_nodes.py
@@ -306,6 +306,11 @@ def test_f_strings(self):
         ast = abuilder.string_build(code)
         self.assertEqual(ast.as_string().strip(), code.strip())
 
+    @staticmethod
+    def test_as_string_unknown() -> None:
+        assert nodes.Unknown().as_string() == "Unknown.Unknown()"
+        assert nodes.Unknown(lineno=1, col_offset=0).as_string() == "Unknown.Unknown()"
+
 
 class _NodeTest(unittest.TestCase):
     """test transformation of If Node"""

'AsStringVisitor' object has no attribute 'visit_unknown'
```python
>>> import astroid
>>> astroid.nodes.Unknown().as_string()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tusharsadhwani/code/marvin-python/venv/lib/python3.9/site-packages/astroid/nodes/node_ng.py", line 609, in as_string
    return AsStringVisitor()(self)
  File "/Users/tusharsadhwani/code/marvin-python/venv/lib/python3.9/site-packages/astroid/nodes/as_string.py", line 56, in __call__
    return node.accept(self).replace(DOC_NEWLINE, "\n")
  File "/Users/tusharsadhwani/code/marvin-python/venv/lib/python3.9/site-packages/astroid/nodes/node_ng.py", line 220, in accept
    func = getattr(visitor, "visit_" + self.__class__.__name__.lower())
AttributeError: 'AsStringVisitor' object has no attribute 'visit_unknown'
>>> 
```
### `python -c "from astroid import __pkginfo__; print(__pkginfo__.version)"` output

2.8.6-dev0

Thank you for opening the issue.
I don't believe `Unknown().as_string()` is ever called regularly. AFAIK it's only used during inference. What should the string representation of an `Unknown` node be? So not sure this needs to be addressed.
Probably just `'Unknown'`.
It's mostly only a problem when we do something like this:

```python
inferred = infer(node)
if inferred is not Uninferable:
    if inferred.as_string().contains(some_value):
        ...
```
So for the most part, as long as it doesn't crash we're good.
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