If you "lock horns" with somebody you argue with them about something.
This photo is from 2-dog-farm on Flickr (reproduced with permission). It's two cows locking horns over something - probably who should get the juiciest grass.
This photo is from 2-dog-farm on Flickr (reproduced with permission). It's two cows locking horns over something - probably who should get the juiciest grass.