There's a common assumption that if the word begins with "al-" in Portuguese, it probably came from Arabic.
Consider the following examples:
- alarm (alarme)
early 14c., from Old French alarme (14c.), from Italian all'arme "to arms!" (literally "to the arms").
1777, from Spanish or Portuguese albino, from Latin albus "white" (see alb)
- algae (alga)
1794, from alga (singular), 1550s, from Latin alga "seaweed," which is of uncertain origin, perhaps from a PIE root meaning "to putrefy, rot."
- allergic (alérgico)
1911, from allergy + -ic; perhaps modeled on French allergique (1906)
- allusive (alusivo)
c. 1600, from Latin allus-, past participle stem of alludere + -ive
- alluvial (aluvial)
1802, from Latin alluvius "alluvial" (from alluvium: "matter deposited by flowing water," 1660s, from Medieval Latin alluvium, neuter of alluvius "washed against," from Latin alluere "wash against," from ad- "to, against" (see ad-) + -luere, comb. form of lavere "to wash").
- ally (aliado)
late 13c., (...) from a differentiated stem of aliier (from Latin alligare "bind to;")
- alter (alterar)
late 14c., from Medieval Latin alterare "to change"
- allumn (ex-aluno)
1640s, from Latin alumnus "a pupil," literally "foster son," vestigial present passive participle of alere "to nourish"
These are spread throughout several native European languages and their likely origin is Latin (was Latin influenced by Arabic?)...
Not focusing on nouns used for names of places
and particular (older) nouns (e.g.: alface), how many words in Portuguese starting with "al-" really have Arabic origins? Perhaps answering this will provide some basis for an answer to an harder question about Arabic influence in Portuguese...