digyoudaoicibaDictYouDict[dig 词源字典]
dig: [13] The origins of dig are not altogether clear. It does not appear to have existed in Old English, although it has been speculated that there was an Old English verb *dīcigian, never recorded, derived from dīc ‘ditch’ (the standard Old English verbs for ‘dig’ were delfan and grafan, whence modern English delve and grave). Another theory is that it was borrowed from Old French diguer ‘make a dyke, hollow out the earth’. This was a derivative of the noun digue ‘dyke’, which itself was borrowed from a Germanic source that also produced Old English dīc (and indeed modern English dyke).
=> ditch, dyke[dig etymology, dig origin, 英语词源]
dig (v.)youdaoicibaDictYouDict
early 14c. (diggen), of uncertain origin, perhaps related to dike and ditch, either via Old French diguer (ultimately from a Germanic source), or directly from an unrecorded Old English word. Native words were deolfan (see delve), grafan (see grave (v.)).

Slang sense of "understand" first recorded 1934 in Black English, probably based on the notion of "excavate." A slightly varied sense of "appreciate" emerged 1939. Strong past participle dug appeared 16c., but is not etymological. Related: Digging.
dig (n.)youdaoicibaDictYouDict
late 17c. as "a tool for digging," from dig (v.). Meaning "archaeological expedition" is from 1896. Meaning "thrust or poke" (as with an elbow) is from 1819; figurative sense of this is from 1840.