Abstract: In recent years, the word“xiaoku”(laughing cry) has gained its popularity worldwide. In fact, it is different from the meaning embodied by the expressive symbols. Of the word, the part“ku”(cry) has developed into a new polar de- gree complement. As a polar degree complement, it as its unique semantic and grammatical features. A diachronic study of it has shown that“ku” was originally a verb and the terminus of the feeling burst and has evolved into the meaning of the polar degree through metaphor. It has the dynamic complexity, fuzzy authenticity and collocative flexibility. The structure of“X-ku”requires that X expresses the speaker’s aesthetic experience and subjective assessment. The cross-lingual study proves that “ku” expresses the extremely high degree and has its own significance of typology.