The term TFAS was coined around 2007 by the member Merchelo(gg etc), and has since gained popularity among the regulars.
A TFAS is usually declared in #lobby or other SRS BSNS channels when a user expresses their need and intention to excrete human faeces.
TFAS, in it's original form, stands for:
Time For A Shit.
TFAP: Time For A Piss: Fairly easy to understand, a user expresses his/her intention to urinate, and goes and does so.
TFAF: Watch out for this one. It stands for Time for a FAP. (FAP FAP FAP FAP etc.)
It is generally accepted that when members return from a TFAS, that the TFAS is measured on the http://en.wikipedia.org/wiki/Bristol_Stool_Scale.
In the summer of 2008, lithium and Zyox investigated the weights of fecal matter. To weigh your crap, simply weigh yourself before and after your tfas, and subtract the two to find the weight of the turd.