名詞 fastness:

1. a rate (usually rapid) at which something happens
   同義詞：speed, swiftness
2. the quality of being fixed in place as by some firm attachment
   同義詞：fixedness, fixity, fixture, secureness
3. a strongly fortified defensive structure
   同義詞：stronghold

1. We guarantee the fastness of these dyes.
   我們擔保這些染料不褪色。

fast.ness

n.（名詞）
The condition or quality of being fast, especially:
迅速：快的狀態(tài)或品質(zhì)，特指
Firmness; security.
堅固；安全
Rapidity; swiftness.
快速；快捷
The quality or condition of color retention; colorfastness.
不褪色性：顏色保持的品質(zhì)或狀態(tài)；不褪色性

A secure or fortified place; a stronghold.
要塞：安全或設(shè)有防御工事的地方；堡壘
A remote, secret place.
偏遠的，秘密的地方