The two of them are inseparable, together all day long.
Tips
history
From 《史记·鲁仲连邹阳列传》 (Records of the Grand Historian): the original line compares deep bonds to the way 胶 (glue) and 漆 (lacquer) fuse - once joined, they cannot be separated. Most often used today for romantic couples.
usage
Almost always describes lovers or newlyweds, not friendships. For platonic closeness use 形影不离 (shadow-and-body inseparable) instead.
