Can I get a coffee? Instead Can I have a coffee , that seems to be an Americanism or is it juts bad grammar?

I was on holiday once and an American in a bar asked a friend how long we were on holiday for, to which the English chap replied 'A fortnight'. The American asks 'What the hell is a fortnight' I had to explain to him in was 14 days and a shortening fourteen nights. He said ' Jeez you guys sure do have a strange language'!! Oh the irony.