Minuki is a Japanese slang term meaning "to masturbate to something".
It's also a meme originating from an image of a conversation between two Ragnarok Online players, which can be viewed here. The image consists of a male player asking a female player if he can masturbate to her avatar. Below is the translation of the full conversation.

♂：Um, sorry, but I have a request.
♀：What is it?
♂：Would you please let me minuki (masturbate to your character)?
♀：Minuki？
♂：Yes.
♀：Oh...
♀：So that's what it is...
♂：So, may I?
♀：Hmm.
♀：Should I say stuff like "you're so pent up"?
♂：Yes.
♀：I guess I have nya choice... (in the original image, the female player ends her sentence in the verbal tic "nya", the Japanese onomatopoeia for a cat meowing, presumably to be cute)
♀：Go ahead.

This tag is to be used when the above copypasta or otherwise reference to the act of minuki is featured in an image; however, this does not mean masturbation in general--only when the phrase 見抜き or its variants are involved.