Of any generation!  In fact, these two references are both to Bullroarer Took, once as Old Took’s g-g-uncle and once as Bilbo’s g-g-g-g-uncle.  I have chosen to make these one entry, but let the debate rage on!  Neither form, nor great-uncle, nor grand-uncle, appear in the Project Gutenberg 100K most common words.

“Grand-uncle” is apparently more common in Ireland than “great-uncle” which is elsewhere the more popular form.

• 01.094 even to Old Took’s great-grand-uncle Bullroarer,
• 01.097 I had a great-great-great-grand-uncle once,

“grand-uncle, n.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/80718. Accessed 13 September 2017.