How do you get ab/oblique muscle fast?

[Rexian]

I don't want a visible ab, but i want ab muscle due to i have none, it's the reason on why my rib cage show and the reason my stomach is so thin (i lost my ab muscle from losing muscle at a deficit). But what's the fastest or a decent way to get ab muscle? or obliques?

(ps: what is that exercise called where you stand up and you swing your upper body side to side with dumbbells and it works the obliques)

[Rexian]

bump

[Backfire16]

if you have a weak core just start out with planks, side planks, exercise ball crunches. The exercise you're asking about is the dumbbell side bend.

[SuperHippo]

eat in a surplus and do ab exercises done.

[Jderavian]

Train your abs for years