sparing no effort; taking pains to do
A set phrase meaning to do something without begrudging the effort, often used in formal or written contexts to describe diligent, painstaking work.
He spared no effort and continued his research late every day.
Thanks to the painstaking preparation, the presentation was a great success.
A set phrase combining 労 (effort, labor) with the negative continuative form of 惜しむ (to begrudge, to spare), literally 'without begrudging effort'.