機密 (kimitsu: secrecy; highly classified information) occasion + secret
The kanji 機 has a plethora of meanings, as the following breakdowns show:
machine
機械, kikai: machine
machine + device, instrumentaircraft
機首, kishu: nose of airplane aircraft + neck
opportunity, occasion
機会, kikai: opportunity
opportunity + opportunity
危機, kiki: crisis danger + opportunityfunction (e.g., of the mind)
機能, kinō: function, faculty
function + functionloom (with the kun-yomi of hata)
機屋, hataya: weaver loom + shopkeeper
When it comes to 機密 (kimitsu: secrecy; highly classified information), I’m not exactly sure which meaning 機 has (as Halpern doesn’t say), but I’ve taken a stab at it and chosen “occasion.” If you feel that I’m wrong, I hope it’ll be our secret!