1 also flare up [intransitive]
to suddenly begin to burn, or to burn more brightly for a short time:
The fire flared up again.
2 also flare up [intransitive]
if strong feelings flare or flare up, people suddenly become angry, violent etc:
Rioting has flared up in several towns.
Tempers flared during the debate.
3 also flare up [intransitive]
if a disease or illness flares up, it suddenly becomes worse:
The injury has flared up again, keeping him out of today's game.
4 [intransitive and transitive]
if a person or animal flares their nostrils (=the openings at the end of the nose), or if their nostrils flare, their nostrils become wider because they are angry:
The bull flared its nostrils.
5 [intransitive always + adverb/preposition]
if a piece of clothing flares out, it becomes wider at one end
The dress flares out from the hips.