Tuneless means having no pleasant tune — music or singing with no melody you can follow, or none worth following. It is the direct opposite of tuneful, and it is nearly always a criticism. Two situations bring it out. The first is someone who cannot sing: a tuneless voice wanders off the notes, so what comes out is not really a melody at all. This use is often affectionate rather than cruel — you say your father sang a tuneless version of "Happy Birthday" and everyone smiles, because being unable to sing is a normal, harmless human thing. The second is music a listener finds empty: a critic calls an album tuneless when the songs never settle into a melody the ear can hold on to. That use is genuinely negative. There is a third, quieter use worth knowing, because it surprises students: tuneless can simply mean without any particular tune, with no criticism at all. "He gave a tuneless whistle and walked away." Nobody is being insulted there — the writer just means the whistle was not any recognisable song, it was the aimless sound a person makes when they are thinking about something else. This is very common in novels, usually with whistle, hum, or humming. So read the sentence before you decide the word is an insult. The adverb is tunelessly, the noun tunelessness, and both are much rarer than the adjective.

不成調的、沒有旋律的、五音不全的。講人唱歌就是「五音不全、走音」，通常帶點好笑或無奈，不算太傷人；講一張專輯 tuneless 就是真的在批評，說它沒旋律、不好聽。還有一種用法沒有褒貶：小說裡寫 a tuneless whistle，意思只是「隨口吹了幾聲，不成曲調」，並不是說吹得難聽。