Papermaking term. In French the same word, “main” or ‘hand’ is used for the bulk of the paper and a quire of 25 sheets. Originally, paper folders counted 5 sheets with each finger on a hand, hence the units of paper (one ream = 20 “hands” = 500 sheets).

By extension, a paper that is very thick in spite of a low mass is said to have “main” meaning bulk.

The “hand” is a way of quantifying the feeling of the thickness+volume pair by touch.
Cotton paper has more bulk than cellulose paper.