IPA: /ˌʌn.kənˈvɪn.sɪŋ/
KK: /ʌnˈkənˌvɪnsɪŋ/
Not able to persuade someone or make them believe something; lacking in convincing power.
The actor's performance was unconvincing, leaving the audience unsure of his character's motives.
Unconvincing → It is formed from "un-" (meaning not) and "convincere" (meaning to conquer or overcome). The word describes something that does not succeed in persuading or convincing someone.
Think of something that is 'not' ('un-') able to 'overcome' or persuade you ('convincere') — that's why unconvincing means it fails to convince.