Pronunciation: /ˈhä ˌhä, ˌhä ˈhä/
noun
a ditch with a wall on its inner side below ground level, forming a boundary to a park or garden without interrupting the view.
Origin:
early 18th century: from French, said to be from the cry of surprise on suddenly encountering such an obstacle

Photo Credit: Andrew Macpherson
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Ling Ling's* attempted escape from Sing Sing
Was thwarted by the ha-ha.
He didn't waste time with pointless hand wring.
He made up a bridge and was off to Baja.
He left them a note,
"Ho, ho. Hee, hee. Gone and free."
About crossing their moat.
The guards felt ditched and didn't get the joke. There was no "Ah, ha!" moment.
*[This is a blatant attempt to panda to the zoo-going crowd.]