Lukewarm beer doesn't taste good.
Standard kanji spelling; 温 is more common than 暖 for this word.
Alternate kanji spelling using 暖; less common but still seen.
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'.