One way to hold this word: someone whose bag is already in the house. The two tell you that a person is not going home at the end of the day, because this is where they sleep. Every use turns on that one fact, and on what it costs: no rent and no journey, but also no line between working and being at home, or between dating and sharing a home. The source records the word simply as live + in, and the sound follows the verb — /lɪv/, not /laɪv/.
可以想成一個人的行李已經在這間屋子裡拆開了。這兩個音節告訴你：他不會在一天結束時回家，因為這裡就是他睡覺的地方。所有用法都繞著這件事，以及它的代價：不用付房租、不用通勤，但也沒有「下班」和「在家」的界線，或「交往」和「住一起」的界線。來源的詞源就寫 live + in，讀音也跟著動詞走，是 /lɪv/，不是 /laɪv/。
From the word’s history
The source's etymology tree gives the word simply as live + in.
來源的詞源樹把這個字直接記為 live + in（住＋在裡面），現代用法就是這個組合搬到名詞前面的結果。
Wiktionary, "live-in"