Helping someone through thick and thin is helping them through the good and bad in life. Although no one is entirely sure which one is the bad and which one is the good out of thick and thin
To be able to last through and handle the good times as well as the bad times. (Usually in the sense of a relationship of sorts). Thick is representative of the good times, whereas thin is representative of the bad times.
throgh thick and thin, if something is THICK then it is HARD to get through. If something is THIN it is EASY to get to. through thick and thin definition= throguh the bad and the good.