(a) To call attention; as, _Hello_, _Ahoy_, _Whoa_.
(b) To express silence; as, _Shh_, _Hush_, _Hist_.
(c) To direct or drive out, etc., as, _Whoa_, _Gee_, _Haw_, _Scat_.
4. +Words used to imitate sounds made by animals, machines, etc.+, as,
_Bow-wow_, _Ding-dong_, _Bang_, _Rub-a-dub_.
When we wish to imitate noises or sounds made by animals, machines,
etc., in writing, we spell out the words as nearly as we can, just as we
write _ding-dong_ to represent the sound of the bell or _tick-tock_ to
indicate the ticking of a clock.
Note that a number of our verbs and nouns have been formed from
imitating the sound which these nouns or verbs describe or express, as
for instance, _crash_, _roar_, _buzz_, _hush_, _groan_, _bang_, _puff_,
etc.