atestar

To pack, cram, or fill something to capacity — atestar una maleta, la plaza estaba atestada de gente. It's most often seen as the past participle atestado/a describing a place jammed full of people or things, more than in its finite conjugated forms.

• El metro iba atestado en hora punta. — The subway was packed at rush hour.
• Atestaron el camión de cajas hasta el techo. — They crammed the truck full of boxes up to the roof.
• La plaza estaba atestada para el concierto. — The square was jam-packed for the concert.

Assuming atestar only means 'to testify' because it looks like atestiguar — its dominant modern sense is physical crowding or packing full; a rare, distinct formal-legal sense of atestar meaning 'to attest' does exist, but atestiguar is what native speakers actually use for courtroom testimony.

Often confused with atestiguar.