noun which may take the genitive case particle 'no'
cheapest; lowest-price
Used attributively with の to mean 'the cheapest' or 'lowest-priced', often in advertising or comparison contexts.
This store offers the cheapest prices.
最安のプランを選びました。
I chose the lowest-price plan.
Compound of 最 (sai, 'most') and 安 (yasu, 'cheap'). The reading さいやす is a straightforward combination of the on'yomi of 最 and the kun'yomi of 安.