mouth
口 centers on the mouth as an opening: the physical mouth, and by extension any opening or entrance that resembles one.
mouth
population
account (e.g. bank)
exit; way out
entrance; entry; gate; approach; mouth
counter; window; teller window; ticket window
(verbal) abuse; speaking ill of people; bad-mouthing; name-calling; slander; abusive language; insulting remarks
(verbal) abuse; name-calling; abusive language; insulting remarks
mouth of river; estuary
tone (of voice); manner of talking
modus operandi; criminal technique; trick
aperture; bore; calibre; caliber
word of mouth
pet saying; favorite phrase; stock phrase; catchphrase
west entrance; west exit
east entrance; east exit
excuse; pretext
big mouth; mouth opened wide
taciturn; reticent; quiet; silent; untalkative
quarrel; argument; row; (verbal) dispute
lipstick
meddling (in); butting in; interference; poking one's nose into
north entrance; north exit
oral; verbal; spoken; parol
口 is a pictograph of an open mouth. The modern square form preserves the simple outline.
The square shape looks like an open mouth. Picture a wide-open mouth saying 'ahh'.
For コウ, imagine a mouth opening wide to say 'ko' as in 'koala': ko -> コウ, and the mouth shape matches the square.