noun
something that lasts a lifetime; high-quality lifelong item
Colloquial noun, often used for durable goods, tools, or products of exceptional quality that one expects to use for life. The suffix もん is a casual contraction of もの.
See also: 一生モノ
この包丁は一生もんだよ。
This kitchen knife is something that will last a lifetime.
いい家具は一生もんになるから、多少高くても買う価値がある。
Good furniture becomes a lifelong item, so it's worth buying even if it's a bit expensive.
一生もの is the standard, non-contracted form. 一生もん is a more casual, colloquial variant with the same meaning.
Compound of 一生 (いっしょう, 'lifetime') and the colloquial suffix もん, a contraction of もの ('thing').