[Unix] On-line acronym for ‘Use the Source, Luke’ (a pun on Obi-Wan Kenobi's “Use the Force, Luke!” in Star Wars) — analogous to RTFS (sense 1), but more polite. This is a common way of suggesting that someone would be better off reading the source code that supports whatever feature is causing confusion, rather than making yet another futile pass through the manuals, or broadcasting questions on Usenet that haven't attracted wizards to answer them.
Once upon a time in elder days, everyone running Unix had source. After 1978, AT&T's policy tightened up, so this objurgation was in theory appropriately directed only at associates of some outfit with a Unix source license. In practice, bootlegs of Unix source code (made precisely for reference purposes) were so ubiquitous that one could utter it at almost anyone on the network without concern.
Nowadays, free Unix clones have become widely enough distributed that anyone can read source legally. The most widely distributed is certainly Linux, with variants of the NET/2 and 4.4BSD distributions running second. Cheap commercial Unixes with source such as BSD/OS are accelerating this trend.