Unreasonable means going beyond what is fair or sensible. Asking for more than anybody could give.
It is a workhorse of complaint and of law, since an unreasonable demand, unreasonable delay and unreasonable behaviour are all recognised phrases, and the last was a ground for divorce in England for decades. The word tests conduct against what a reasonable person would do, which is one of the oldest ideas in English law. It is not the same as irrational, since an unreasonable request may be perfectly logical and still unfair.
1adjectiveunreasonableGoing beyond what is fair, sensible or acceptable.
“It is not unreasonable to expect an answer.”
Latin
irratiōnābilis
Middle English
unresonable
English, today
unreasonable
From Middle English unresonable, equivalent to un- + reasonable, originally partly after Latin irratiōnābilis.
Etymology from Wiktionary, CC BY-SA