Can also be attributed to females in regards to her metaphorical dick, most commonly considered derogatory if used by a male unless one of these conditions apply:
-Said female is a lesbian (thus referring to her strap-on).
-Said male is in the friend zone and the girl will disregard his statement as he is a non-entity.
-Said female is, in fact, a transexual, or hermaphrodite.
-Said female is a honey badger
Layman's terms: Thinking with one's dick, or popping a boner or erection in public. Sporting a metaphorical woody.
Dave: "I can't help but salute every hot chick that walks by! I've been walking on all threes the entire day!"
Sandy: "If I had a dick, I'd be walking on all threes for her."
Friend Zone Fred: No, I totally wouldn't, I'm only interested in you.
Sandy: Aw, how sweet, you're such a good friend.
Friend Zone Fred:(thinking to himself) Fuck.
(What Friend Zone Fred should've said) "What are you a lesbian? Kinky." "Fuck yeah, hold on, lets meet up later." "Yeah, I can be tectonic friends with her." "Reminds me, if we screwed I'd have you walking on all four's the next morning" Etc... Pretty much any option or combination thereof that'll probably get him smacked, anything but option A.
Seriously, don't be Fred.