Drunk, or at least well on the way, from drinking beer. It is slang, and quite informal — you hear it at parties, in pubs, in casual talk between friends, mostly from British speakers, though Americans understand it too. It almost always appears as 'get beered up' or 'be beered up'; you will rarely see 'beer up' as a plain verb. It usually describes a group of noisy, cheerful (or rowdy) drinkers rather than one quietly drunk person — 'beered-up football fans' is the classic picture. Too casual for writing or for talking to a teacher or boss.