The first-tone liē only appears doubled inside fixed expressions: 大大咧咧 (carefree, sloppy in manner) and 骂骂咧咧 (to grumble curses while doing something). It is never used on its own.
Left mouth radical, a small square picturing parted lips. It indexes 咧 in the speech-and-mouth family: 喊, 叫, 吃. Here it specifically marks the lips being pulled wide into a grin.
Right side 列 supplies the sound: liè drifting to liě in 咧嘴. The same phonetic powers 烈, 裂, 例. Faint semantic flavour too: when you grin your teeth show in a tidy 'row' under stretched lips.
