General: reverse trap

Similar to trap, a female character who looks or acts masculine, likely dressing in male attire. Unlike mere crossdressing, the character is assumed by others to be male until demonstrated otherwise.

As a rule of thumb, if you can correctly tell the gender with complete certainty from the thumbnail alone, it's not a trap. Also, posts containing a reverse trap do not require the male tag.

(Reference: Danbooru)
Updated by nokill over 1 year ago