guard is not 121.5
On a MIL comm set, a function allows a second freq to be monitored (RECEIVE ONLY) in addition to the active transmit/receive frequency. This function is called Guard. You can guard any frequency you want to besides just 121.5 - eg company or a VOLMET or ATIS freq.
They call it guard for obvious reasons because it's usually left on 121.5/243.
Guard is not the name of 121.5 or any other frequency despite the prodigious use of the term by civvy pilots. The same people who aren't disciplined enough to resist the temptation to repeat "You're on guard" ad nauseum because they think it's funny appear to be the same ones who are clueless about the term "guard" itself.
If some people need a good reason to keep their traps shut on 121.5 or why this is a good subject, they can keep in mind the tower will transmit direct to an aircraft on this 121.5 during a loss of comm as will APP and other ATS agencies.
A lot of pros keep 121.5 selected on COMM 2/R for this reason and really grind their teeth when listening to idiots kicking off as they're on final.