"When examining phonetic evolution in historical linguistics, assimilation and dissimilation are two crucial concepts. This paper focuses on dissimilation, particularly “bilabial dissimilation”. The sound change known as “dissimilation” occurs when two similar sounds in a word become less alike over time. Reduplication is a common word -formation process in many languages. In monosyllabic languages, reduplication forms include patterns such as [AA], [ABB], [AAB], [AABB], [ABAC], and others. This study investigates three vernacular expressions in modern Taiwanese Southern Min that were formed through reduplication, analyzed from the perspective of labial dissimilation. The three examples are as follows:
1. kui5 sɔm1sɔm1/kui5 sam1 sam1/kui3 saŋ1 saŋ1, meaning “expensive”. It is proposed that sɔm1/sam1/saŋ1 derives from the morpheme “森” .
2. put7 tap7 put7 tshit7, meaning “not good” or “not presentable”. Commonly written as “不答不七”, its etymology is argued to be “不八不七” (put7 pat7 put7 tshit7).
3. put7 tian1 put7 kai5, meaning “improper, unbecoming, and inappropriate behavior”. Its etymology should be “不邊不界” (put7 pian1 put7 kai5).
All three vernacular words involve reduplicative, specifically the [ABB] and [ABAC] patterns. This paper argues that in “森” (sɔm1), labial dissimilation occurs within the syllable, whereas in “不八不七” and “不邊不界,” the dissimilation is triggered by the presence of three consecutive syllables beginning with bilabial voiceless stops, causing the second syllable to change to a non-bilabial initial consonant. The discussion reveals that labial dissimilation n functions both intra-syllabically and inter-syllabically in quadrisyllabic formations. Recognizing the effects of this phonological process in Taiwanese Southern Min vernacular enables us to identify the etymologically accurate characters that align in both pronunciation and meaning, thereby allowing for a more precise analysis of the structural and semantic features of these expressions.