「Orifice in high tension, distorted, pulled open not in the normal shape」or「mouth during a medical examination pulling one's rolled skin into second position」as a GRIMACE, or BRAGGING about one's healthy condition, or SPEAKING LIES about the cause of a problem made manifest by pulling back the skin.「Rod orifice misshapen」described another activity a person cannot do by himself, that of reaming the urethra to treat a gonorrheal infection: GRIMACE. Additional minor Chinese meaning is HOLLERING, HOWLING like a wolf under a full Moon—and why is that, do you suppose?
Alternative glyph is「heavenly orifice, mouth」(吴) which ought to provide clues as to overall usage. Meaning DOING SOMETHING FOR SOMEONE ELSE, DOING SOMETHING FOR ANOTHER THAT THEY CANNOT DO FOR THEMSELVES. What is something that a man (in this case) cannot do for himself, but would like to do for himself, that would cause a「mouth to become distorted in high tension for a long time causing gasps, and with both hands」? and also meaning to GIVE? nothing expresses the kindness, gentleness, and selflessness *joy* of GIVING better than「giving head.」The ability to perform autofellatio is present in only two or three male humans in a thousand, indicating that natural selection has reduced this allele within the human population considerably but not entirely, yet, most probably because those males able to perform such a feat spent the entire day doing so, leaving themselves open to predators that may attack while the subject is wholly self-absorbed and defenseless (data for female humans is unavailable, since perhaps females simply do not attempt this act as often as do males. Population sample excluded circus performers, wrestlers, and gymnasts).
Scholars add these other meanings of this glyph: DISTORTED SPEECH, to SPEAK LIES, to BRAG. All may be related to a man talking about his ability to actually give himself a blow job by distorting himself (not a joke: search 'autofellatio' if interested).

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: area comprising southern Jiangsu, northern Zhejiang and Shanghai; name of states in Southern China at different historical periods
Unihan extended: one of warring states