The plural showing duplicated daleth, which is from Mishnaic Hebrew, may originally have been an Aramaism there. See also s.vv. ṣyd n.m.#3 and prep. for more frequent inner Aramaic developments.
For BA see s.vv. lcd prep., mcd prep.
The cited example of בצד is the only case where it is found in JBA outside of Hebrew contexts.
Page refs. in other dictionaries:
DJPA: 458aDJBA: 228bDrower/Macuch: 393a, 385bDNWSI: 959