Understand your fellow gymgoers. The following are the first few entries in a crude dictionary I'm putting together of fitness lingo.
Jacked Big and muscular.
Damn... I just saw a picture of the Cowboys' Terrell Owens with his shirt off and that dude is jacked."
"Sean eats seven meals a day; he's getting jacked."
Origin: It probably derives from the image of jacking up a car--pumping your arms looks like you're inflating yourself. 
Synonyms: diesel, pumped, and buff (although most guys don't say "buff" anymore). Similar to but not exactly synonymous with "yoked" (see "yoked" for more.)
Antonyms: small, stringy, puny, Urkel.
Editor's note: My friend and colleague, master trainer Joe Stankowski, once remarked to me that he had never heard the term "jacked" before he started writing for MF. I asked him what the midwest slang for a big guy was. He said, "big."
Ripped Having a high degree of muscle definition. More so than one who is "cut", but not as lean as one who is "shredded", such as a pro bodybuilder.
"Brad Pitt sure was ripped in Fight Club." 
"Sean is jacked and ripped--that's so hot."
Origin: Probably derives from the idea that a ripped person looks like he or she has had all the body fat ripped off, leaving only pieces (muscles) behind. 
Synonyms: chiseled.
Antonyms: fat, Gordita Supreme, John Goodman.
Meathead A dumb, insensitive clod who wears a wifebeater undershirt to the gym and whose life revolves around lifting weights, or at least banging them loudly on the floor. Also known to hit on various female gym members with lines such as "How you doin'?", and "lemme show you how to tone up your arms." He's an idiot, but can usually kick your ass.
"I wish that meathead would stop grunting, because I can't hear my workout mix."
"Sean is so jacked and ripped, you'd think he was a meathead, but he's not."
Origin: Big, muscular guys tend to resemble sides of beef. While being stupid is not a pre-requisite for being strong, strong guys often act dumb or belligerent when their testosterone is racing.
Synonyms: meatwad, dumb jock, Greg Tolan (brilliantly played by William Zabka) in the 80s classic, Just One of the Guys.
Antonyms: nerd, brain, CNN financial correspondent.
Yoked A big, muscular guy with a particularly distinctive trapezius and neck.
"That guy looks like he has no neck; he's yoked."
"Sean is jacked, ripped, and yoked, but he's not a meathead--he's amazing."
Origin: Football player or wrestler types like this look like they're wearing the yoke farmers put on oxen to pull heavy machinery.
Synonyms: no-neck, bad ass, "my bad for being in line before you got here; go right ahead"
Antonyms: skinny, scrawny, Andy Dick.