Tunny is an older British word for the fish we now nearly always call tuna. Same fish — the big sea fish that gets cut into steaks or packed in tins. The thing to know is that this word is fading. In Britain today you would order tuna, buy a tin of tuna, eat a tuna sandwich; 'tunny' turns up in older books, in fishing talk, and sometimes in the fuller form 'tunny fish', but a young person in London may never use it. Americans do not use it at all. So treat it as a word to recognise rather than a word to say. If you meet it in a novel or an old cookbook, just read it as 'tuna' and you have lost nothing. The plural can be 'tunny' or 'tunnies', which itself tells you how unsettled and old-fashioned the word is.

鮪魚（英國舊式說法，現在幾乎都講 tuna）。看得懂就好，不用拿來講。