Having expectations or ambitions that are unrealistically high; aiming too high. Used as a noun and as the verb '高望みする'.
給料も経験もないのに、いきなり管理職を望むのは高望みだ。
Wanting a managerial position right away without pay or experience is aiming too high.
夢は大きく持つべきだが、現実的な計画を立てることも必要だ。単なる高望みでは意味がない。
You should dream big, but it's also necessary to make realistic plans. Mere wishful thinking (aiming too high) is meaningless.
彼は高望みするあまり、小さな成功を認められなかった。
He aimed so high that he couldn't appreciate small successes.