From Old English 'laenan' (to grant), from PIE *leykw- (to leave) — lending was originally leaving something with someone.
To grant temporary use of something on the condition that it or its equivalent will be returned.
From Old English 'lǣnan' (to lend, grant, lease), from 'lǣn' (a loan, gift, grant), from Proto-Germanic *laihwniz (a loan, something left with another), from PIE *leykʷ- (to leave, to remain, to be left over). The word's deep history connects lending to leaving: to lend was to leave something with someone temporarily, with an expectation of return. The PIE root *leykʷ- is also the source of Greek
The '-d' at the end of 'lend' is not original — Old English had 'lǣnan' with no 'd' at all. The consonant was tacked on during Middle English by analogy with 'send,' 'bend,' and 'rend,' and the same process gave English 'sound' (from Old French 'son') and 'bound' (from Old Norse 'búinn').