noun
weather vane; weathercock
A device, often shaped like a rooster, that shows wind direction. The literal, physical object.
屋根の上に風見鶏が立っている。
A weather vane stands on the roof.
noun
fence-sitter; opportunist
Figurative use for a person who changes their opinion or allegiance according to the prevailing situation, like a weather vane turning with the wind.
彼は風見鶏だから、どっちの意見にもすぐ同調する。
He's a fence-sitter, so he quickly agrees with whichever opinion is popular.