What is the meaning of whirlpool tattoo?

It can differ. To some people it can be the start of a sleeve with water, where water means the ultimate equalizer (in military), water can splash and crash but never puts force against force (in fighting). Water equalizes every man, no matter how tough they are, the tough becomes aligned with everyone else (in water operations/missions). Advanced fighters of jiu jitsu or specificly brazilian jiu jitsu or grappling will be looking for to get their game flowing like 'water' (not just sitting stuck fighting in one position with strength). They will also want to move fluidly like water, not flail around.

To others it can be a spiral that goes inwards or outwards which can be negative or positive.