also: あさまら
noun
morning wood; morning erection
Vulgar slang term for a morning erection. Used in casual, often male-oriented conversation.
朝起きたら朝マラが立ってた。
When I woke up, I had morning wood.
朝立ち is a more neutral, clinical term for morning erection, while 朝マラ is vulgar slang.
Compound of 朝 (asa, 'morning') and マラ (mara, vulgar slang for 'penis'). The exact origin of マラ is uncertain, but it is a longstanding slang term.