na-adjective
lukewarm; tepid; unpleasantly warm
Describes something that is mildly warm, often with a negative nuance of being not hot enough or unpleasantly warm, such as a bath that has cooled down.
風呂が生温かくなってしまった。
The bath has become lukewarm.
生温かいビールは美味しくない。
Lukewarm beer doesn't taste good.
Also means lukewarm, but is a more common and neutral i-adjective. 生温かい often carries a stronger negative nuance of being unpleasantly warm.
Compound of 生 (nama, 'raw, fresh, unprocessed') + 温かい (atatakai, 'warm'). The prefix 生 adds a nuance of 'half-' or 'not fully', giving the sense of 'not properly warm'.