Step 1: Sterilize five half-pint jars and their lids in boiling water; keep them warm in the water until ready to fill.
Step 2: In a large, heavy-bottomed stockpot, combine the mashed bananas, undrained crushed pineapple, lemon juice, sugar, and shredded coconut.
Step 3: Bring the mixture to a full boil over medium-high heat, stirring constantly to prevent the sugar from sticking or burning.
Step 4: Once boiling, reduce the heat to medium-low and simmer for about 20-25 minutes, stirring frequently, until the mixture thickens and reaches a jam-like consistency.
Step 5: Ladle the hot butter into the warm jars, leaving exactly 1/4 inch of headspace.
Step 6: Wipe the rims of the jars with a clean, damp cloth and secure the lids and bands until finger-tight.
Step 7: Process the jars in a boiling water bath for 10 minutes (adjust for altitude if necessary).
Step 8: Carefully remove the jars and let them cool on a towel-lined counter for 24 hours without moving them.