1) bowels, intestines, (the heart, lungs, liver, etc.)
1b) the bowels were regarded as the seat of the more violent passions, such as anger and love; but by the Hebrews as the seat of the tenderer affections, especially kindness, benevolence, compassion; hence our heart (tender mercies, affections, etc.)
1c) a heart in which mercy resides
Part of Speech: noun neuter
Citing in TDNT: 7:548, 1067
Probably strengthened from σπλήν; splên
(the "spleen"); an intestine (plural); figuratively pity or sympathy: - bowels, inward affection, + tender mercy.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root σπλάγχνον; click on the second column to search for that grammatical form of the root σπλάγχνον; click on the third column to search for that word and grammatical form; click on the total to search for the root σπλάγχνον.