A phenomenon which only occurs once an individual wears jeans. It is most commonly used to describe the lump or bulge which forms on the around the zipper/crotch area when sitting down. Any attempt to 'squash' the said boner may result in making the situation even more awkward, leading most to ignore it and hope nobody else notices, much like when you pop a real boner on the train and can't get it to go down.
Fred noticed he was rocking a denim boner but when he tried to push it down, everyone on the train started to stare.
A public erection which is noticed only by close contact (Usually during 'dirty dancing' or 'freaking.') The boner is concealed from sight by thick jeans or slacks and is only identified upon feel/touch, usually when a girl is grinding on a guy while dancing.
"That guy I was just dancing with totally got a Denim Boner!"
"Kelly likes dancing at the club; she enjoys the attention and giving guys Denim Boners."