If the difference between all the military branches is still confusing to you, well, this old meme is an example of Marine humor about airmenpic.twitter.com/fxSMzLOIE9
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Turning it into a code phrase is writers wanting to make the military simultaneously appear more and less regimented than it really is (subordinates are never allowed to give their opinion AT ALL unless they say the magic words, in which case they are protected from all harm)
(I am not a veteran, but what I've heard from hanging around veterans online trying to make my fanfic more authentic is that saying "permission to speak freely" is very Hollywood and the most you'll hear is an NCO going "meaning no disrespect" before saying something to a CO)
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