The only free and comprehensive online etymological dictionary of the Spanish language
acá (Adverb) "here," "hither"
11th cent. From Vulgar Latin *ac(cu) hac from Latin eccum hac "look at this." For a continued etymology of eccum, see aquí. Latin hac "in this manner" is from Proto-Italic *χā-ke "by means of this here." From Proto-Indo-European *geh2- or *ǵheh2-, both meaning "this" with suffix *-ḱe "here."
As a contrastive with aquí, mostly confined to Latin America and only uncommonly found in Iberia. Typically, acá is used with motion verbs (e.g., vienes acá "you come here") while aquí is used for other purposes (e.g., se renta carros aquí "cars are rented out here").
Indo-European
Romance
Asturian acá, Portuguese , Italian qua

Notice that acá's story is one of consistent reinforcement as the word loses semantic strength over time. Originally, Indo-Europeans added *h1e- and *-ḱe to mean "this here." By the time of the Proto-Italic period, the word (now *ek(e)) had weakened in meaning and the Italic people felt the need to reinforce it with the suffix *-ke "here" much as their ancestors had done several thousand years before them. As time went one, the Italic word evolved into Latin ecce and (again) weakened in meaning. The Latin speakers felt the need to reinforce the word with hac, and that eventually became Spanish acá.

Mas acá, meaning "this side" or "this way," derives from its use as an intensifier. Unlike mas allá, which neatly translates as "beyond," mas acá has no isomorphic equivalent.