nursing, my **s

why does one call drinking beer 'nursing a beer'?....the whole thing totally escapes me...one associates nursing as taking care and being extremely caring and careful...what's taking care has to do with beer?...drinking 20 tall ones with friends make you go drunk, depending on your tolerance level, not extremely caring...weird...where does that phrase coined from?...one thing i don't need to think about but i just can't help to...