E. De Felice and E. Rossoni agree that the surname has an anthroponymic origin of Germanic root, derived from the old medieval name Aldemarus or Valdemarus.

E. De Felice notes that it is widespread from the peninsular South (with maximum frequency in Bari) as far as Emilia, but is not very common. He explains that it is a variant of the name of Germanic origin Aldemaro —as shown by the fact that the 17th-century Neapolitan jurist Biagio Altomare or Altomari was actually called Aldimari—, no longer alive in use and with no semantic correspondence, and therefore reinterpreted by popular etymology and deformed into Altomare. He adds that the Germanic name may be a variant or a crossed form of Ademaro, or an adaptation of a Germanic personal name Waldemar, composed of *walda- ("power, dominion") and *maru- ("great, important; outstanding; famous"), meaning "powerful and famous".

E. Rossoni agrees with De Felice in indicating that the surname derives from dialectal modifications of the medieval name of Germanic origin Aldemarus or Valdemarus, known through the king of the Danes and Slavs Valdemar at the beginning of the 13th century, which shows its diffusion in the European sphere.

V. Blunda agrees with the previous authors on the Germanic origin of the surname, but offers a different interpretation of the formative elements. He notes that it is a Sicilian surname derived from the compound Alda Maru, meaning "old and famous" or "illustrious by age", formed from alda (old, aged, wise) and maru (famous, renowned, known for his deeds). He warns that the popular interpretation of the surname as "high sea" is a false etymology, since the original Germanic composition has no relation to the Italian noun. He explains that it is preserved in areas of Lombard imprint in southern Italy.