fundament (n.)

• backside
• base
• basement
• basis
• beam
• bed
• bedding
• bedrock
• behind
• belly
• bottom
• breech
• buttocks
• derriere
• downside
• floor
• flooring
• footing
• foundation
• fundamental
• ground
• grounds
• groundwork
• hardpan
• pavement
• posterior
• principle
• radical
• rear
• rudiment
• rump
• seat
• sill
• substratum
• substructure
• underbelly
• undercarriage
• underside
• understructure