'Welching' is the clunking of the base of a beer bottle or glass, whilst making various grunting or 'pirate-sounding' noises such as arrrrrr, or oarrrrr... etc etc. This is as opposed to 'clinking' or 'chinking' of the top of a bottle, and it perceived as far more masculine when in a group of lads who are all very drunk, and in agreement of a certain point of topic.
It is often accompanied by the raising of one's right elbow upwards, whilst making other peculiar noises, that would suggest that one has slightly lost the plot.
There was some serious welching going on last night!
When in agreement, one member calls out 'welching!' and group members proceed to welch in above said fashion.