Verb (5-dan, む, intransitive)
1. to bite into (e.g. rope into skin); to cut into; to dig into (e.g. of fingernails)
彼の腕に爪が食い込む。Nails dig into his arm.
2. to eat into; to encroach; to make inroads; to penetrate (e.g. a market); to break into (e.g. first place); to erode; to cut into (e.g. time, savings)
上位に食い込んだこともある。There have been times when it broke into the top ranks.
3. to be wedged (i.e. underwear pulled from the back, driving it between the buttocks)
パンツがお尻に食い込んで気持ち悪い。My underwear is riding up my butt, and it feels awful.