Green’s Dictionary of Slang

cheese v.3

also cheeze
[the photographer’s demand that one ‘say cheese’ to produce a smile]

(orig. UK public school) to smile.

[UK]N&Q CLVIII 119/1: Another slang use of the word ‘cheese’ was in vogue at Rugby School... This was with the meaning ‘smile’, both verb and noun.
[UK]M. Marples Public School Sl. 40: Cheese [...] to smile or grin (Oundle 1920+).
[[Aus]A. Buzo Rooted I i: gary Say cheese. bentley Cheese].
[US]Eble Campus Sl. Mar.
[US]R.C. Cruz Straight Outta Compton 12: Yolanda wheled back around and cheesed, playing the nut roll.
[US]Big L ‘Ebonics’ [lyrics] Smilin’ is cheesin’.
[US]Teen Lingo: The Source for Youth Ministry [Internet] cheezin v. Smiling. ‘Why you cheezin so big?’.
[US]J. Díaz This Is How You Lose Her 168: I did it, Paloma says, cheesing.