Although it has always been conceded that "Kalookan" is of purely Tagalog derivation, historians have never really agreed on the true origin of the name.  According to Serrano:

"There are several opinions or beliefs in regards to how this interesting town got its name.  Many believe that Caloocan originated from the Tagalog word look (bay).  Its proximity to Manila Bay and its being located on the eastern fringe of Dagat-Dagatan, a small lake separated from Manila Bay by the municipality of Navotas, gives weight to this belief.  Some believe that formerly the word look meant sulok, or corner.  So, Caloocan might have meant, in the past, na sa sulok, or in the corner.  As a matter of fact, in the past, Caloocan was located "at the corner" where the ends of two old towns, Tondo and Tambobong (Malabon), meet."

Both theories seem readily objectionable.  Kalookan was not a bay town.  It was in fact separated from Manila Bay by Dagat-Dagatan.  On the other hand, sulok or nasa sulok is so far removed from and could hardly have evolved into kalookan; the logical transformation would have been kasulukan.  Besides, sulok could have applied only to the original village in the shores of Dagat-Dagatan, but that settlement was called Aromahan (now Libis).

Dr. Fausto J. Galauran, in his "Ang Kasaysayan ng Simbahang Katoliko sa Kalookan" (1966), suggested that look might have been an old form of loob (interior).  The town site was, in fact, interior territory, if the point of entry were, as it was then, the shores of Dagat-Dagatan.   This theory is bolstered by Director Ponciano Peralta Pineda of the Institute of National Language, who says that there was an old Tagalog word, loog, which used to be synonymous with loob.  An interior territory, then, could have been referred to as kaloogan.  Tagalog phonetics, which allow the substitution of the "k" sound for that of the consonant "g" (as baksak for bagsak) could have led to the evolution of  Kaloogan into Kalookan.

The loog, or kaloogan, was the hill East of the shore of Dagat-Dagatan, where the fishermen-refugees from Tondo had settled.  To the growing Aromahan community, it was the most logical site for expansion.