1/3/5 flop aggression from fish

1/3/5 10 handed

The rock is out everyhand, but played from every single position so the position is weird preflop.

Villain is usually a loose passive fish. He and hero just sat down at the same time(2~3 orbits), tonight he's been aggro so far. Opening like every hand, then double barreling k4s on 522r 9 8 but check back river vs another fish. He bought in for 1000+.

Last session, seen him check/call with fd then hit on river still checking oop. He said he's waiting for his opponent to bet so he can check/call.

Only seen him aggro out of line 1 time. 1 asian guy just sat down with deep stacks, going aggro first hand with ATo preflop vs a shortstacker open + couple callers. The very next hand a guy open 20, fish(villain) calls, 1 other caller asian guy squeezes to 125, initial raiser and this fish call.

flop qj5hh asian guy bets 125, initial raiser calls, villain raises to 300, asian guy 3bets to 700(800~1000 behind), both fold (fish had similar stacks i believe)

Hero has played 2 sessions with him before, this is the 3rd, hero has a tight image.

Effective stacks 420ish.

Preflop
Rock is on utg+1, all folds to V in sb opens to 15, bb flats, Hero in utg 3bets to 55 w/AJo. V calls, BB folds.

Pot 130
Flop ATTss, V checks, Hero bets 40, V check raises to 140, Hero?

What does it even mean to check/raise this flop?

We only have like 330ish behind.