tennis let rules
From the ITF Rule book:The service is a let if:
a. The ball served touches the net, strap or band, and is otherwise good; or, after
touching the net, strap or band, touches the receiver or the receiver’s partner
or anything they wear or carry before hitting the ground; or
b. The ball is served when the receiver is not ready.
In the case of a service let, that particular service shall not count, and the server shall
serve again, but a service let does not cancel a previous fault.
Additional approved alternative procedures can be found in Appendix V.
23. THE LET
In all cases when a let is called, except when a service let is called on a second
service, the whole point shall be replayed.
Case 1: When the ball is in play, another ball rolls onto court. A let is called. The
server had previously served a fault. Is the server now entitled to a first service or
Decision: First service. The whole point must be replayed.
As for the origin of the word:
The one that seems to be most likely is that the word for 'net' in French is 'filet' and since modern tennis has French roots (prior to its current form, there was a similar French game) it's likely 'filet' was modified to 'let' when the British came up with their rules.