From Middle English forwaxen, from Old English forweaxan (“to progress, grow too much, become overgrown”), equivalent to for- + wax (“to grow, increase”). Cognate with German verwachsen (“to grow together, heal over, overgrow”).
forwax (third-person singular simple present forwaxes, present participle forwaxing, simple past forwaxed, past participle forwaxed or forwaxen)
- (intransitive, obsolete) To grow to excess; become huge; overgrow; swell.
1475, Couldrette, The romans of Partenay:
Horrible gret was, A forwoxen grome, Such Another neuer had he sain.