Tunelessly is the adverb from tuneless. It describes HOW someone sang, hummed or whistled: without any pleasant melody. Because it is an adverb it attaches to a verb — "he hummed tunelessly", never "a tunelessly song". In practice this word lives with a very small set of verbs. Look at real writing and you will find it almost always next to hum, whistle, or sing, and most often in novels. "She hummed tunelessly as she washed up." "He whistled tunelessly through his teeth." That is nearly the whole life of the word. Notice what it is doing in those sentences. It is usually not saying the person is a bad singer. It is painting a small picture of someone absorbed in something else — the sound is aimless, a habit, not an attempt at a song. Writers use it to show a character who is distracted, bored, or comfortable enough not to care. When it IS about bad singing, the context makes that clear: "the drunk crowd sang tunelessly all the way home". The opposite is tunefully.

不成調地、沒有旋律地（哼、吹、唱）。小說裡最常見的是 hummed tunelessly，意思接近中文「隨口哼哼」——不是說他唱得難聽，而是那個聲音本來就沒在唱什麼歌，人在想別的事。真的要說唱得走音，也可以用，但要看上下文才知道是哪一種。