That word perplexed me too. Since it is also used in the chapter title, then I think you have to deduce its meaning from the context of the message of that chapter. I found this suggestion from a google search. Seems plausible to me.
http://www.trollope-apollo.com/?p=2776
Quote:
As Reginald and Mary discuss the various social obligations they will have to take on, Reginald suggests to Mary that she should regulate all that she does according to the great doctrine of tanti. Tanti is the genitive singular of the Latin adjective tantus, which means so much. Tanti is being utilized as a genitive of value, which means of so much worth. The word in this form can also be translated as worthwhile. Social customs and expectations can be worth performing even if they are not strictly necessary.
The chapter is entitled Is It Tanti? This could be understood as the question Mary should ask herself about each of her actions as she implements Reginald's doctrine. It could also be a question asked about fox-hunting and its social function. In fact, Reginald introduces the idea of tanti to Mary while they are talking about fox-hunting: Reginald maintains that opponents of the practice do not understand that recreation is as important as the material necessities of life. Trollope closes this chapter by asserting that the day's sport certainly had been tanti, answering his own question. [KS & RR 2012
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