Translation:I was forced to do this against my will.
eh? why is the same word used for to thank and to force? Could someone explain the reasoning behind that?
Because "obrigado(a)" is the short for "eu me sinto obrigado(a) a retribuir o favor que você me fez".
It's the same as the somewhat archaic English expression, "Much obliged!" That also means "Thank you" but it sounds a bit more formal.
Any time you'd use "thanks/thak you" in English, you can use "Obrigado/a".
Think "obligated" or "obliged" as being based off the same Latin root that "obrigado" is. Probably another loaner word from the Roman Empire or the Hundred Years' War between England and France.
Could this also be 'i was forced to make this against my will' ? (for example, slave labour)
come on, I said "i had to do this", I don't mind correction if I can learn something form it. In fact, I look forward to it. But still,,,,come on.
if you say '' I had to do this'' it means eu tive que fazer isso, and that's not translated correctly
However, that is only in the literal sense, and that is not the only way to do translation, nor is it many times the best way. In many cases, DL already demontrates that, though it seems to vary depending on the language. The best way is to indicate the strongest literal translation, while indicating secondary, but also acceptable ones. Here, "obligated" also would be a good equivalent.
Doesn't make sense to me. Why "obrigado". I read the comment below and that doesn't make sense to me either .
Think of the English "obligate". It means to compel or to force. In Portuguese it is "obrigar". When we say "obrigado" in response to a favor, we are saying "I am now obligated to you", or, in more usual terms, "I owe you one". As someone else mentioned, an equivalent English phrase (now out of fashion) is "much obliged". I hope this helps.