The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
Appendix I
Indo-European Roots
ENTRY:
bhergh-2
DEFINITION:
High; with derivatives referring to hills and hill-forts. Oldest form *bherh-, becoming *bhergh- in centum languages. Derivatives include iceberg, bourgeois, burglar, force, and fortify. 1a.barrow2, from Old English beorg, hill; b.iceberg, from Middle Dutch bergh, mountain; c.inselberg, from Old High German berg, mountain; d. Germanic compound *harja-bergaz (see koro-). ad all from Germanic *bergaz, hill, mountain. 2.belfry, from Old French berfroi, tower, from Germanic compound *berg-frij-, high place of safety, tower (*frij-, peace, safety; see pr-). 3. Zero-grade form *bhgh-.a.borough, burg, from Old English burg,burh,byrig, (fortified) town; b.burgomaster, from Middle Dutch burch, town; c.bourg, bourgeois, burgess, burglar; faubourg, from Late Latin burgus, fortified place, and Old French burg, borough; d.burgher, from Old High German burgri, townsman, from Germanic compound *burg-warn-, city protector (*warn-, protector; see wer-4). ad all from Germanic *burgs, hill-fort. 4. Possibly suffixed zero-grade form *bhgh-to-.force, fort, fortalice, forte1, forte2, fortis, fortissimo, fortitude, fortress; comfort, deforce, effort, enforce, fortify, pianoforte, reinforce, from Latin fortis, strong (but this is also possibly from dher-). (Pokorny bhereh- 140.)