'Forbore' is the past tense of the verb 'forbear', though I do believe the first 'e' is supererogatory
It is not an unusual word. For example, consider Mr. Thomas Babbington Macaulay's Horatius on the Bridge
No sound of joy or sorrow Was heard from either bank; But friends and foes in dumb surprise, With parted lips and straining eyes, Stood gazing where he sank; And when above the surges, They saw his crest appear, All Rome sent forth a rapturous cry, And even the ranks of Tuscany Could scarce forbear to cheer.