noun which may take the genitive case particle 'no', noun
extra-thick; heavyweight
Used especially for paper, cardstock, or similar sheet materials that are thicker than standard. Often appears in product descriptions or specifications.
This business card uses extra-thick paper.
Extra-thick envelopes are sturdy and suitable for mailing.
Compound of 特 (とく, 'special, extra') + 厚 (あつ, 'thick') + 口 (ぐち, here a suffix for paper thickness grades).