A massage boner in an uncontrollable erection you get on the massage table if anything or anyone comes even close to your junk. It's unavoidable and potentially embarrassing when this happens, especially if the massage is done by a dude.
Robin: Was so embarrassed last night.
Reg: Why ?
Robin: Man, I went for a massage and it was a dude. Couldn't help getting the hugest massage boner when he rubbed my thighs and his elbow brushed my junk. Wouldn't settle down for 10 minutes.
Reg: Did he finish it off?
Robin: Hell no.
It's code for large breasts. Can be used in the presence of women so they don't know what you are talking about. Or to avoid any Human Resource trouble.
Harry: Yo man,check out that chick. She's got a rich family.
Eddy: No shit. Looks like two puppies fighting in a stocking.
Harry: I'd motorboat that shit in a second.