In article 01c3ced4$a9575f50$0125250a@brwcrsqsezyxmfzy,
says...
Steve Silverwood ...

^ ... ALWAYS monitor the GUARD frequencies ...

Emergency frequencies are guarded, or watched, therefore they are often
spoken of as guard frequencies. "Guard" is a verb in this case, not a noun.
When units are transiting in company with each other only one unit need
maintain the guard, and that unit is referred to as having "the guard". So
when "guard" is used as a noun it refers to the unit maintaining the guard --
the unit watching the emergency frequencies. Through misunderstanding this
term has become somewhat corrupted and I often hear sheriff aviation units
calling 121.5 the guard channel.

Sorry for the late reply.

The terms "GUARD Channel" and "GUARD Frequency" have been used for at
least 30 years. They were used as such in USAF technical orders for
various avionics communications systems, which I serviced while I was in
the USAF. Flight crews frequently use the phrase, "listening on GUARD"
to refer to monitoring 121.5 and 243.0 while in flight.

So the case can be made for either usage.

--

-- //Steve//

Steve Silverwood, KB6OJS
Fountain Valley, CA
Email: