Trousers that aren't actually slack. In fact they're more like drainpipes. There was a strange schoolboy trend to wear them as fashion items and must-haves in comprehensive schools of the 1980s, but now only appear to be sold via those crap catalogues or whole-page adverts you get in the magazines that accompany the Sunday papers.
Their biggest claim to fame is that they don't require ironing and come in the most god-awful shades of grey, brown, and blue. Made of plastic (rather like Barrack Dress trousers), they are identified by the little brown label with the curly "F" on them. The only people who wear them now are old men and RAF officers who seem to think them stylish.
An accessory to Farah pants were Kicker shoes/boots and Pringle, Lyle and Scot jumpers. Poor kids could be identified by not havig 'F' on their pants, and C&A shoes with the (by now) Gucci leather tag hanging from the laces. Semi hip dudes would have Fred Perry jumpers.
No Farah pants and Kickers usually meant free dinners and an unstable family background. Farah Slacks are thus a key RAF signature equipment.
There is another similar item of "lower body apparel" called No2 Trousers (Lightweight). These were introduced in the mid-'80s by the Assistant Director of Clothing Requirements and Procurement (ADCRAP - noduf) and have the ability to look like Farahs but, additionally, crease like a granny within 2.5 seconds of being removed from an ironing board. This contributes to the rather crumpled RAC Patrolman appearance of the modern RAF, and is why aircrew are reluctant to remove their growbags.