Referring to someone as being 'stim' is, for want of a better word, to describe them as being one boring bastard! i.e. the antithesis of stimulating.
'Stim' can also be used to describe an activity or group of individuals - e.g. "My God the chess club are a stim bunch of cnuts!"
Stims are also refrigerated soft drinks - though this terminology is/was usually confined to tropical postings when every block has/had a fridge-full of stims - paid for on 'tick' from the camp boon or char wallah. These were a positive life saver for a drunk & dehydrated wreck after a session at the local atap bar.
Stimms - meaning stimulants.