When a younger person is confronted, with complete surprise, by a naked middle aged person or senior in a manner that is akin to walking into the locker room at a country club.
Whoa, I just got country clubbed by my father-in-law. Eeeugh.
When a guy is turned on, but not quite ready to grab a couch pillow to awkwardly conceal his arousal- an erection that is safe for public consumption- i.e. not pitching a tent, but beginning to feel aroused.
When that girl with thatfine ass bent over right in front of me to adjust the spoke nipples on her Fuji bicycle, I chubbed up a bit.