What is "Thud"?

Good question.  For one, it's unique and relatively easy to remember.   Not like there's 
a thousand "House of Thud"s out there.

To me though, it's an onomatopoeia that can mean the sudden arrival of a strange but perhaps useful thought.
"I was just sitting there then *thud*, I realized how it all worked."
Or the state of being here now, and the stickiness of the sound.  Something that
goes *boing* has bounced away..  something that goes *splat* hit way too hard and left a mess,
but *thud* is just right.   No rebound, no mess, just a state of being at the point
of impact.  Being here, now.    (Yes, I'm making it all up.)

Other than that, it represents the sound you feel in your body from some of my
favorite things.     The thud of thunder in your body that's so close that the
flash and boom come nearly at the same time.   Feeling a large explosion through
the ground before the sound reaches your ears (I hate how the movies and TV always
sync up the sound with the sight.   It should be vision first, then shockwave through
the ground, then sound).     Big drums..   Koto, Jimbe, marching bass drums and
so forth.   I like a drum that can literally take your breath away.  

Thud is whatever you want it to be.