Internet/youth slang. Note 凹 here is read āo (the same character that means 'concave' in 凹凸) - a borrowing from Cantonese 'āu' meaning 'to make/strike (a pose).' Has a slightly mocking flavor when applied to others.
memory
Picture the character 凹 as a pose with a notched-in waist - fits the meaning visually.
