A thicko is a stupid person — but the word matters much less for its meaning than for its flavour, so let me be careful here. It is British, informal, and it is an insult. It belongs to playground talk, tabloid newspapers, and TV comedy, not to anything you write or say in an office. Americans mostly do not use it at all; they would say 'idiot' or 'dummy'. Behind it is British 'thick' meaning stupid ('he's a bit thick'), plus the ending -o that English tacks onto words to make a casual, slightly rude nickname — the same -o as in 'weirdo' and 'sicko'. That -o is the whole personality of the word: it makes it sound like something shouted rather than considered. How rude is it? Softer than the strongest insults, harder than 'silly'. Between friends teasing each other it can be almost affectionate — 'you thicko, that's not how it works' — but said to a stranger, or about a real person's ability, it is cruel, because it attacks the person rather than the mistake. This is why I would tell you to recognise it and not to use it: you will meet it in British comedy and comment sections, and you should know you are reading an insult. It is also the kind of word whose sting a non-native speaker judges wrong, because tone is doing most of the work.

笨蛋、白痴（英式粗俗用語，帶侮辱性）。台灣講的「笨蛋」語氣其實比它輕，thicko 更接近罵人「腦袋裝漿糊」，看得懂就好，別自己拿來用。