also: いるかせ
na-adjective
Describes a negligent or careless attitude, often in the set phrase ゆるがせにする (to neglect, to treat lightly). Usually used as ゆるがせに.
See also: 忽せにする
彼は仕事をゆるがせにしている。
He is negligent about his work.
安全対策をゆるがせにしてはいけない。
You must not be careless about safety measures.
na-adjective
Archaic sense meaning easygoing or relaxed; not used in modern Japanese.
古語では「ゆるがせ」に「のんきな」という意味があった。
In classical Japanese, ゆるがせ had the meaning of 'easygoing'.
怠慢 is a more formal noun meaning negligence or neglect, often used in official contexts, while ゆるがせ is a more literary adjectival noun used in set phrases.
いい加減 can mean irresponsible or half-hearted, similar to ゆるがせ, but is more common in everyday speech and has additional meanings like 'moderate' or 'enough'.
The origin is uncertain; it may be related to the classical verb ゆるぐ (to slacken) or the adjective ゆるい (loose), but the exact derivation is unclear.