agachar

To bend down, duck, or lower a part of the body — usually reflexive agacharse: se agachó para atarse el zapato. Agachar la cabeza (to bow/lower one's head) extends figuratively to submitting or accepting something without protest, much like English 'to keep your head down.'

• Se agachó para recoger las llaves. — He bent down to pick up the keys.
• Agacha la cabeza, hay una rama baja. — Duck your head, there's a low branch.
• Prefirió agachar la cabeza antes que discutir con el jefe. — He chose to keep his head down rather than argue with the boss.

Forgetting the reflexive se for the literal physical action — agacharse (to duck/bend down oneself) is what a learner needs for describing their own movement; agachar without se is transitive, to lower something else (agachar la cabeza is the fixed idiomatic exception).

• expression "Agachar la cabeza" — A common fixed expression meaning to submit, accept humiliation, or give in without protest — literally 'to lower one's head' as a gesture of submission or shame, the Spanish counterpart to English's 'to keep one's head down.'