ROMÂNÉSC, -EÁSCĂ, [Var.: rumânésc, -eáscă adj.] – Român + suf. -esc.
-ÉSC suf. lat. -iscum
This issue is indeed discussed in the histories of the Romanian language and it is, as I recall, fairly well explained.
In all the languages spoken in ancient Balkans there was this suffix –iscos (lat. –iscus, gr. –ισκός, tdm. –isco/-ēsko), so formally it may come from latin. Nevertheless, it is Al. Graur (if I am not wrong) the one who noticed that the lat. –iscus, as well as the gr. –ισκός are diminutival suffixes, having another function than rom. –esc(u). For this reason he proposed a multiple etymology: from tdm. –isko and from latin –iscus. Al. Graur is the author of the "multiple etymology theory", which says that a language element (not only words, but suffixes, prefixes, grammatical elements etc.) can come from more than one language. So, for example, the rom. suf. –ţie comes from Russian. The word revoluţiune comes from the French révolution. So, revoluţie comes partly from French, partly from Russian; it has a multiple etymology. The same happened to the lat. –iscus which became –esc(u), but under the influence of the dacian –isko.
Now, in Romanian this is a suffix which creates adjectives, denoting most of all the origin (of a person). With this function it became the most usual suffix to create personal names (which soon became family names), mainly from other names. Thus, the surname Buză became Buzescu, Petre > Petrescu, Popa (from the appelative popa) > Popescu a.s.o.
Its plural was used to denominate a family or a group of families (a clan) bearing this name: Buzeştii, Popeştii etc. This plural is the normal evolution in Romanian of the group of sounds –sci (-sce), as in lat. piscem > *pesče > peşte, broscae > broasče > broaşte etc. The same way Popescu, Ionescu in the plural became first Popesci, Ionesci (and the old Romanian orthographic forms were indeed so) – pronounced Popesči, Ionesči – and then the group –sč- finally turned into –şt-.
All toponyms in –escu and –eşti come from family names in –escu: Bucureşti from a Bucurescu (created from a name Bucur), Ploieşti from Ploiescu (< Ploaie, a surname) a.s.o.
