na-adjective, noun
unpromising; not to be relied on; of little hope
Describes a situation, prospect, or person that offers little expectation of success or reliability. Often used in the structure 期待薄だ or 期待薄の.
彼の成功は期待薄だ。
His success is unlikely.
この計画は期待薄の見通しだ。
This plan has an unpromising outlook.
薄い means 'thin' or 'weak'; in 期待薄 it functions as a suffix indicating scantiness of hope.
Compound of 期待 (きたい, 'expectation') and 薄 (うす, 'thin, weak'). The exact historical derivation is uncertain, but the word follows a common pattern of noun + 薄 to express 'little of something'.