This can be loosely used to describe a mood, situation or routine behaviour (usually social).
2) (verb); to act out these behaviours or moods.
This is often associated with (but not limited to) exhibiting dominance in prowess of either physical strength (e.g. muscle size by FLEXING your arms) or cultural knowledge - namely dressing and dancing to impress and demonstrating a superior capability of use of language.
This is usually a playful display and is a part of innocent courtship, although it can be taken to mean a more sinister and aggressive stance.
1) "I was ballin all evening with my homies until we hit the club flex to find some fly bitches"
"That mad cat was on a mental flex, kicking off at blokes in the street if they so much as looked at him"
"The author of this article was on a bored flex at the time of writing"
2)"I was flexing with this chick till her man comes along and drags her off"
"I like to flex when I've had a sip or two of yak"