This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
靣 appears to be a variant or older form of 面, but the exact historical development is uncertain.
The shape resembles 面 (face, surface), but the top is slightly different. Think of it as a rare variant of 面.
For メン, remember it sounds like 面 (メン), the common kanji for face: 面 -> メン, and 靣 is its rare variant.