The character 乙 is a pictograph of a bent or coiled object, such as a hook or a fish's gut. This suggests something curved or winding, which can relate to its use as the second in a sequence, following a straight 'first'.
The shape of 乙 looks like a stylish, flowing letter 'Z'. Think of 'Z' as 'second' in an informal alphabetical order, or as representing something 'chic' and 'quaint' with its unique curve.
Vocabulary
乙女おとめ
maiden, young girl
甲乙こうおつ
A and B, superiority or inferiority
乙な味おつなあじ
quaint taste, refined taste
乙号室おつごうしつ
Room B (second room)
乙おつ
Cheers!, Good work! (colloquial shorthand for お疲れ様)