In part, that is attributable to the designation of his correspondents, who keep him informed on language that is not within earshot, as the  Lexicographic Irregulars,  an amusing reference the first time or two it was used but now beginning to cloy.
( ( In part ) ( , ( that ( ( is ( attributable ( to ( ( ( ( the designation ) ( of ( his correspondents ) ) ) , ) ( who ( keep ( him ( ( ( informed ( on ( language ( that ( ( is not ) ( within earshot ) ) ) ) ) ) , ) ( as ( ( ( ( ( the ( Lexicographic Irregulars ) ) , ) ( ( an ( amusing reference ) ) ( the ( first time ) ) ) ) or ) ( two ( it ( was ( ( used but ) ( now ( beginning ( to cloy ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (NN part))) (, ,) (NP (DT that)) (VP (VBZ is) (ADJP (JJ attributable) (PP (TO to) (NP (NP (DT the) (NN designation)) (PP (IN of) (NP (PRP$ his) (NNS correspondents))) (, ,) (SBAR (WHNP (WP who)) (S (VP (VBP keep) (SBAR (S (NP (PRP him)) (VP (VBD informed) (PP (IN on) (NP (NP (NN language)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (RB not) (PP (IN within) (NP (NN earshot)))))))) (, ,) (PP (IN as) (NP (NP (DT the) (NNP Lexicographic) (NNP Irregulars)) (, ,) (NP (NP (DT an) (JJ amusing) (NN reference)) (NP (DT the) (JJ first) (NN time))) (CC or) (NP (NP (CD two)) (SBAR (S (NP (PRP it)) (VP (VBD was) (VP (VP (VBN used)) (CC but) (VP (ADVP (RB now)) (VBG beginning) (S (VP (TO to) (VP (VB cloy)))))))))))))))))))))) (. .)))
The Lexicographic Irregulars were an amusing reference at the beginning.
( ( The ( Lexicographic Irregulars ) ) ( ( were ( ( an ( amusing reference ) ) ( at ( the beginning ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNP Lexicographic) (NNP Irregulars)) (VP (VBD were) (NP (NP (DT an) (JJ amusing) (NN reference)) (PP (IN at) (NP (DT the) (NN beginning))))) (. .)))