To get (very) drunk
Also 'to go on the lash'.
Synonymous with:
Get pissed, get wasted, get
hammered, get rat-arsed, get sloshed, get trolleyed etc
The word 'lash' orinally means to strike with a whip, and so has this
extended meaning (ie. drinking makes you feel like you've been flogged).
NB. Usually used in a positive sense.