lest conjunction
Is 'lest' an old conjunction which is not much used in nowadays? Or is it a very common conjunction used also in day to day speaking?
"every bath.. my ears are stretched its a must do, lest they be stinky."
I found its meaning from internet but still doubt whether it's an old word. Otherwise it would be a very useful word for me when to refer first meaning of below two.

Meanings found in internet:
1. With the intention of preventing (something undesirable); to avoid the risk of.
2. (after a clause indicating fear) Because of the possibility of something undesirable happening; in case.

Question

lest conjunction 
Is 'lest' an old conjunction which is not much used in nowadays? Or is it a very common conjunction used also in day to day speaking?
"every bath.. my ears are stretched its a must do, lest they be stinky."
I found its meaning from internet but still doubt whether it's an old word. Otherwise it would be a very useful word for me when to refer first meaning of below two.

Meanings found in internet: 	
1. With the intention of preventing (something undesirable); to avoid the risk of.
2. (after a clause indicating fear) Because of the possibility of something undesirable happening; in case.

Answer

Your understanding is excellent. The word is not used in ordinary speech, but more formally in writing. As you say, it is a useful word, although perhaps a bit old-fashioned.

ご自宅で快適に語学を学べるチャンスをお見逃しなく。経験豊富な語学講師陣の中からお選びいただき、今すぐ最初のレッスンにお申し込みください！