From bound +‎ -less.

boundless (comparative more boundless, superlative most boundless)

1. Without bounds, unbounded.
   • 1785, William Cowper, “The Garden”, in The Task, a Poem, in Six Books. By William Cowper […] To which are Added, by the Same Author, An Epistle to Joseph Hill, Esq. Tirocinium, or a Review of Schools, and The History of John Gilpin, London: Printed for J[oseph] Johnson, No. 72 St. Paul's Church-Yard, OCLC 221351486; republished as The Task. A Poem. In Six Books. To which is Added, Tirocinium: or, A Review of Schools, new edition, Philadelphia, Pa.: Printed for Thomas Dobson, bookseller, in Second-street, second door above Chestnut-street, 1787, OCLC 23630717, page 87:

     'Tis the cruel gripe, / That lean hard-handed poverty inflicts, / The hope of better things, the chance to win, / The wiſh to ſhine, the thirſt to be amus'd, / That at the found of Winter's hoary wing, / Unpeople all our counties, of ſuch herds, / Of flutt'ring, loit'ring, cringing, begging, looſe, / And wanton vagrants, as make London, vaſt / And boundless as it is, a crowded coop.

• bottomless, extentless, limitless, unbottomed, unbounded; see also Thesaurus:infinite

• limited