What I was initially thinking as I read the beginning of your post was that the animal was under-fed were it was being kept and was in a semi-weakened state when you got it, therefore it had no energy to puff up or tail whip you or anything. Then as I read the rest of your post you said the Tegu as skinny, this just re-affirms my theory.
I wouldn't worry about it though, Mig(my tegu) was very docile in my local reptile store and was a little on the skinny side and only puffed up a little bit when he was handled, after i got him home and got some good food in him, he started to puff up alot and whip me, but with some gentle handling for abot 15 minutes each day he has stopped whipping me, although he still puffs up, but i personally think it's cute lol. Once he is out of his enclosure however he is fine, he "hisses" for lack of a better word at 3:38AM a little bit but that's about it, just keep working with him, you might want to shorten your handling sessions a little bit though, but then again the tegu could be fine...
Good Luck!
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0.0.1 Argentine Black and White Tegu
0.0.1 Baby Aru Green Tree Python
2.1 Bearded Dragon
0.0.4 Hatchling Bearded Dragons
1.1 Honduran Milksnake
1.0 Florida Brooksi Kingsnake
1.0 Blood Red Cornsnake
1.0 Jungle Carpet Python
0.1 Desert Kingsnake
1.0 Western Hognose Snake
1.0 Bull Mastiff