1Lukewarm; tepid — used when something that should be hot has cooled to an unsatisfying in-between temperature. Common for tea, coffee, soup, beer, and bath water.
お茶がぬるくなっちゃったから、入れ直すね。
The tea has gone lukewarm, so I'll make a fresh pot.
お風呂のお湯がぬるくて、ぜんぜんあったまらない。
The bathwater is lukewarm and doesn't warm me up at all.
夏のビールはキンキンに冷えていないと、ぬるくてまずい。
Summer beer is no good if it isn't ice-cold — lukewarm beer tastes awful.
レンジで温め直したけど、真ん中がまだぬるい。
I reheated it in the microwave, but the middle is still lukewarm.
2Lenient, lax, soft — of an attitude, rule, discipline, or response. By extension, in casual usage, can describe a competition, test, or environment as 'too easy / not challenging enough'. When written in kanji, usually 緩い (which can also be read ゆるい with the same meaning).
親のしつけがぬるいと、子どもはわがままに育ってしまう。
If parents' discipline is too lax, children grow up spoiled.
そんなぬるい考え方じゃ、この業界では生き残れないよ。
With that kind of soft thinking, you won't survive in this industry.
今回の大会はレベルがぬるくて、物足りなかった。
This tournament was too easy — it left me wanting more of a challenge.