Its a sauce that contains lettuce and vinegar to eat with roast dinner and can be eaten with chips and cold meat in Newcastle, UK.
I believe you are referring to coleslaw - it's actually cabbage rather than lettuce, and is considered a side-dish. It's often served with chips (French Fries here in the US) as an accompaniment to hot and cold roast meats as well as batter-fried dishes such as chicken, clams or fish. Coleslaw dressing can be creamy or vinegary in the US, and it's largely a regional difference - in Southern Cuisine, coleslaw dressing is primarily vinegar, northern recipes call for mayo or sour cream. Reviewing some recipes online, in the UK it appears that coleslaw tends to have both creamy and vinegary components to the dressing.
Do you mean "liquor"? Liquor is a parsley sauce to which vinegar is often added. It's usually eaten with pie and mash.
This is something I made every sunday as a child for our sunday dinner. My children now make the salad for our roast on a Sunday. 1 full soft leaf round lettuce chopped almost until it turns to mush, Half to one whole onion finely chopped, enough malt vinegar until it covers the onion and lettuce in a bowl, then enough sugar to make it sweet and not acidic ( quite a lot!) . glug of boiling water to dissolve the sugar and leave to stand...the longer the better.Served with a hot sunday roast over everything! Great even later on poured over a cold Yorkshire pudding....Sundays aren't the same without it.