As a neutral-tone lie the character is a colloquial, regional sentence-final particle close to 了 or 啦, common in northern speech as a cheerful 'will do!' 好咧. It is informal and does not appear in formal writing.
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.
