also: コしょく
noun
eating only one's favorite food; unvarying diet; poor eating habits
Describes a monotonous diet where someone eats only what they like, often implying nutritional imbalance or pickiness. Can be used critically or self-deprecatingly.
彼は固食で、毎日ラーメンばかり食べている。
He has an unvarying diet and eats nothing but ramen every day.
固食は健康に良くないと分かっているけど、好きなものしか食べられない。
I know a monotonous diet isn't good for my health, but I can only eat what I like.
偏食 is the more common word for 'unbalanced diet' or 'picky eating', while 固食 is a less frequent synonym with a nuance of sticking to one's favorites.
Compound of 固 (fixed, unchanging) and 食 (eating). The exact origin is uncertain, but it likely arose as a straightforward description of a fixed, unvaried diet.