Monumental Etymology: OED

Monumental, adj. and n.[ classical Latin monumentlis (1st cent. A.D.) < monumentum MONUMENT n. + -lis -AL suffix1. Compare French monumental relating to monuments (1802), of majestic grandeur (1805). Compare slightly earlier MONUMENTARY adj.]

1. a. Serving as a monument or memorial.
Quotes: 1596 C. FITZ-GEFFREY Sir Francis Drake sig. E8, The dangerous straights that yet doe bear his name, Are monumentall annals of the same. 1872 ‘M. TWAIN’ Roughing It lxxi, His second in command..opened fire upon the swarms of natives on the beach, and one of his cannon balls cut this cocoanut tree short off and left this monumental stump standing. 1897 H. E. CHETWYND-TAPYLTON Stapeltons of Yorks. 191 Torre also saw four monumental slabstones. 1954 S. PIGGOTT Neolithic Cultures Brit. Isles v. 132 At Wayland's Smithy..monumental peristaliths of standing sarsen stones..surrounded the mound

{dag}b. That serves as a token or reminder; spec. acting as a proof of identity. Obs.
Quotes: a1616 SHAKESPEARE All's Well that ends Well (1623) IV. iii. 18 Hee hath giuen her his monumentall Ring

2. Of or relating to a monument or memorial structure, or to monuments in general. In early use: {dag}of or relating to the tomb; sepulchral (cf. MONUMENT n. 1) (obs.). Also fig.
Quotes: 1603 T. POWELL Vertues Due sig. C6, The hallowedst mold, Left for monumentall vse. a1616 SHAKESPEARE Othello (1622) V. ii. 5 Skin..smooth, as monumentall Alablaster. 1646 R. CRASHAW Steps to Temple 35 Softly may he be possest, Of his monumentall rest

3. Resembling or suggestive of a monument; like a monument, esp. in being large, solid, imposing, etc.