An abstrusity is something which is abstruse - concealed, hidden, or difficult to understand.
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 9 words with the pattern --ST-I-G-. That is, nine letter words with 3rd letter S and 4th letter T and 6th letter I and 8th letter G. In alphabetical order, they are: eastlings hustlings jostlings lustrings nestlings restringe restrings rustlings unstrings
Ten words of ten letters are:acceptancebeauticiancreativitydictionaryefficiencyfertilizergenerationhorizontalimpatiencejeopardize