F.N.R.B. - A boner that hits you when you least expect it, like your pa

A boner that hits you when you least expect it, like your pants are being haunted.

I was walking down the street and my boner was like, 'I'm coming with you.'

I was at the grocery store and my boner was in the cereal aisle.

I was talking to my friend and my boner was doing a TikTok dance.