The sense of deliberate ‘done intentionally’ is older than the closely related, but slightly differently pronounced, deliberate ‘to engage in careful consideration’. The first is medieval, the second from the mid 16th century. Both go back to a Latin word formed from libra ‘scales’, which captured the idea of weighing something up before coming to a conclusion.
前缀de-在此用于加强语气；词根liber-来自拉丁语libra（天秤；衡量），“天秤”是用于“衡量”重量的；所以该词中的-liber-本义是“衡量”，经过仔细权衡的是“深思熟虑的”。“天秤”趋向于平衡，所以有同源词level（水平；等级）。