Tuna (Thunnus spp.): a large, commonly eaten fish used in sushi, sashimi, and canned products; often refers to Pacific bluefin tuna.
寿司屋でまぐろの刺身を頼んだ。
I ordered tuna sashimi at the sushi restaurant.
スーパーで新鮮なまぐろが安く売っていた。
Fresh tuna was on sale cheaply at the supermarket.
まぐろの缶詰は保存食として便利だ。
Canned tuna is convenient as a preserved food.
Slang/vulgar: a derogatory slang term comparing someone to a 'tuna'—used to describe someone who is inert or unresponsive during sexual activity. It is vulgar and offensive.
彼はベッドでまぐろみたいに動かなかったと言われた（卑俗な表現）。
It was said he lay like a 'tuna' in bed (a vulgar expression).
Slang: someone who is unconscious or completely passed out from drinking; a person who is limp and unresponsive due to alcohol.
昨夜の飲み会で彼は飲みすぎてまぐろ状態になっていた。
At last night's drinking party he drank too much and was out cold.